I doubt that the fiat currencies would disappear completely. There are too many countries and too many banking and financial institutions who hold a large amount of sway about the direction of financial transactions that it is almost impossible to erase them! What may happen is that they merge the cryptocurrencies with the fiat to utilize the best of both worlds and earn money out of it just like they do now with transaction fee for the moeny we use Too much power is in very few hands and democracy being a myth a always will never let this happen where people will have distributed power!!